Refactor Future constructors.

tests/standalone/io/io_sink_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 class TestConsumer implements StreamConsumer {
@@ skipping 16 matching lines @@
     return stream.fold(
         received,
         (list, value) {
           list.addAll(value);
           return list;
         })
         .then((_) {});
   }
 
   Future close() {
-    return new Future.immediate(null)
+    return new Future.value()
       .then((_) {
         if (closePort != null) closePort.close();
         Expect.listEquals(expected, received);
         if (expcetedAddStreamCount >= 0) {
           Expect.equals(expcetedAddStreamCount, addStreamCount);
         }
       });
   }
 }
 
@@ skipping 52 matching lines @@
     controller.close();
   }
 }
 
 void main() {
   testClose();
   testAddClose();
   testAddStreamClose();
   testAddStreamAddClose();
 }